Increasing concerns about biological hazards of minute quantities of lead have recently resulted in downward revision of national standards for toxicity levels and have introduced simultaneously a need for sensitive biomarkers of very low body burdens. Pyrimidine 5'-nucleotidase is extremely susceptible to heavy-metal inactivation, especially when compared to a companion erythrocyte isozyme, deoxyribonucleotidase. Preliminary studies indicate that these isozymes might provide useful indicators of biological burdens equivalent to 10-20 micrograms lead/dl of whole blood or less. The principal goal of this proposal is to assess the value and feasibility of using these nucleotidase isozymes as sensitive and reliable indices of low lead overburden in humans. To achieve this goal, studies are designed to determine: (1) relative sensitivities of erythrocyte 5'-nucleotidase isozymes to heavy-metal inhibition and the effects of concurrent sulfhydryl protectors, chelators, and other modifying agents; (2) correlations between quantitative activities of these isozymes and conventional criteria for toxicity such as free erythrocyte protoporphyrin and blood lead concentrations in the range of 0-30 microgram/dl and above; (3) the value of nucleotidase activities and isozyme activity ratios in detecting minimal lead toxicity and in monitoring variations in lead body burdens due to therapy, natural attrition or re-exposure; and (4) feasibility of developing nucleotidase assay technology to rapid small-volume methods practical for population screening. Experimental design to achieve the first objective will rely on in vitro incubation systems utilizing normal human erythrocytes in comparison to cells from subjects with severe hereditary deficiency of pyrimidine 5'- nucleotidase. Blood samples obtained from children's lead screening clinics and from retired lead-industry workers will provide a resource of specimens with very low peripheral blood lead concentrations to fulfill the second objective. Continuation of follow-up studies initiated over three years ago in collaboration with Children's Hospital of Michigan will allow periodic evaluation of therapeutic responses required by the third objective. The fourth objective will involve assessment of existent analytical techniques to develop a screening test for the broadest range of lead exposures based on their differential effects on nucleotidase isozyme activities.